Hillia Jacq. includes about 25species of succulent, usually epiphytic shrubs and small trees distributed widely in the New World tropics. Hillia plants are found in wet tropical forests, from low to montane elevations. The genus can be recognized by its combination of well developed, interpetiolar, oblanceolate to obovate stipules that are held erect and flattened together in bud, its succulent leaves with the secondary venation usually not visible, its usually relatively large corollas with well developed tubes and 4-10 lobes that are convolute in bud, and its cylindrical woody capsules that contain numerous flat papery seeds with a tuft of silky filaments 1-3 cm long on one end. The seeds are apparently wind-dispersed; a similar arrangement, with a tuft of trichomes at one end, is found in one other genus of neotropical epiphytes, Didymochlamys Hook.f. The flowers vary from salverform to broadly tubular, and range from diurnal and red to nocturnal and white, green, or yellow-green. At in least the species with white nocturnal flowering the flowers last for one night. This genus belongs to the tribe Hillieae.
Hillia parasitica is the most commonly collected species.

Species of Hillia can be confused with those of Cosmibuena Ruiz & Pav.; however Cosmibuena differs in its stipules that are shortly united around the stem (vs. separate in Hillia) and its seeds without Hillia's tuft of filaments. Hillia was monographed by Taylor (1994), who included in it the Central American genus Ravnia (Taylor, 1989) and recognized five subgenera. Hillia subg. Hillia is found throughout most of the range of the genus and characterized by fragrant noctural flowers that are pollinated by hawkmoths, and have white salverform corollas with five to six lobes and exserted stigmas. Hillia subg. Illustres is found from southern Central America to southeastern Brazil and characterized by apparently noctural flowers pollinated by bats or perhaps moths, with green or dull orange to purple funnelform corollas with generally six lobes. Hillia subg. Andinae comprises one species of the central Andes with small leaves and flowers, and has flowers similar to those of Hillia subg. Hillia except the stigma is included and positioned near the base of the corolla; this species may also be pollinated by hawkmoths but little is known. Hillia subg. Tetrandrae is found in the Antilles and Central America and characterized by fragrant noctural flowers that are pollinated by hawkmoths, with salverform white corollas with four lobes. Hillia subg. Ravnia is found in southern Central America and northwestern Colombia and characterized by apparently diurnal flowers that are pollinated by hummingbirds, with funnelform to swollen corollas with generally six lobes. One species of Hillia subg. Ravnia, Hillia triflora, is unusual in the genus in its inflorescences that usually have two to three cymose flowers and its corollas that are swollen, with the base and mouth narrower than the middle.